Ultrasonic techniques are of prime importance in the field of non-destructive testing of materials as they enable evaluation of some of their properties and particularly detection of discontinuities present therein. The object testing techniques especially concerned are those which utilize two distinct transducers which are both situated to the same side of the object to be examined, one transducer acting as a transmitter and the other as a receiver, between the transducers there being provided an ultrasonic wave propagation which is variable in conformity with the presence or absence of discontinuities. These techniques are referred to as pitch and catch techniques and are classified in two categories which are referred to as direct and indirect categories.
According to the direct technique, illustrated in FIG. 1 in which E.sub.1 designates the transmitter transducer, the receiver transducer R.sub.1 is arranged in the location in which the ultrasonic beam reflected by the back of the object M being tested is actually expected in the absence of any discontinuity in this object. In the presence of a discontinuity, however, the ultrasonic beam F is interrupted and the failure of reception (by the transducer R.sub.1) of a reflected beam, or in any case a strong disturbance of this beam, constitutes the indication of detection of a discontinuity D. According to the indirect technique, illustrated in FIG. 2 in which E.sub.2 designates the transmitter transducer, however, the receiver transducer R.sub.2 is arranged in the location in which the ultrasonic beam is expected if it is reflected by a discontinuity D.
A test method which involves mainly the diffraction of the ultrasonic beam and operates by observation or measurement of the times of flight associated with various trajectories of the ultrasonic waves combines the characteristics of the two foregoing techniques. According to this hybrid method (referred to as TOFD or Time Of Flight Diffraction), illustrated in FIG. 3, the waves of the diverging ultrasonic beam emitted by the transducer E.sub.3 either directly reach the receiver transducer R.sub.3 (lateral wave O.sub.A which is practically parallel to the surface of the object tested) or undergo a diffraction effect because of the presence of a discontinuity D in the material (waves O.sub.B, O.sub.C diffracted by the extremities of the discontinuities), or reach the transducer R.sub.3 after reflection from the opposite wall of the material (reflected wave O.sub.D). The representation of the temporal position of the signals received by the transducer R.sub.3 thus demonstrates, as shown in FIG. 4, the existence of a discontinuity inside the material: actually, on the time axis of FIG. 4 there are successively encountered a signal S.sub.A which corresponds to the lateral wave O.sub.A, two signals S.sub.B, S.sub.C which correspond to the two diffracted waves O.sub.B, O.sub.C, and a signal S.sub.D which corresponds to the reflected wave O.sub.D (in the absence of any discontinuity, exclusively the signals S.sub.A and S.sub.D would be received).
If the distance separating the transmitter transducer E.sub.3 and the receiver transducer R.sub.3 is referred to as 2S, the depth of the end of the discontinuity causing diffraction is denoted by the reference d, the lateral distance between this extremity E.sub.x and the median plane P situated at the same distance from the transducers is referred to as X, the respective distances between the extremity and the transducers E.sub.3 and R.sub.3 are referred to as M and L, and the ultrasonic speed (for example, in millimeters per second when the other distances are given in millimeters) is denoted by the reference c, the time T necessary for the ultrasonic wave to be diffracted by the extremity E.sub.x and subsequently received by the transducer R.sub.3, i.e. for travelling the path (M+L), is given by the expression (1): ##EQU1## The derivative of this expression (3) in relation to the variable X shows that the time T is minimum (and hence the detector signal is maximum) when X=0, i.e. when the extremity E.sub.x considered is in the median plane P situated at equal distances from the two transducers. Thus, carrying out the TOFD method consists in adjusting, once the signals S.sub.B, S.sub.C indicating the presence of a discontinuity have been detected, the positions of the transducers E.sub.3, R.sub.3 (with a constant spacing 2S) so as to minimize the time T (i.e. the signal received is maximum).
The described TOFD method is disclosed, for example in "Vessel nozzle inner radius examinations using ultrasonic time-of-flight diffraction (TOFD)" by D. F. Loy and J. A. Vano issued at the "Vessel and Internals Inspection Conference" held in San Antonio (Tex., United States of America), Jul. 11-15, 1994.
The execution of this method, however, implies mobility of the transmitter-receiver assembly relative to the object to be tested by sliding on the surface of the latter in the plane of incidence containing the ultrasonic waves shown in FIG. 3, adjustment of the position of the transmitter and receiver transducers not being possible in the absence of such mobility. Unfortunately, the mechanical scanning required to realise this adjustment limits the data acquisition rate, prolongs the testing of objects of large surface area, and makes it more costly. The document U.S. Pat. No. 4,497,210 describes a phased array ultrasonic testing apparatus in which, in order to make easier the mobility, the mechanical scanning is replaced by an electronic one implemented on a single array probe. However, the distance between the transmitter transducer and the receiver one is not constant. Moreover, such mobility is nevertheless difficult to realize inside tubular objects to be tested.
In the course of such an adjustment the position of the transducers must be marked by means of localization means such as a position encoder. Actually, such localization is necessary if for easier interpretation of the test results for the object examined it is desired to form an image in which, for example one of the marking axes corresponds to the time of flight whereas the other axis, extending perpendicularly to the first one, corresponds to the position of the transmitter and receiver transducers, the amplitude of signals on that image being encoded in grey levels. Thus, the precision of any mechanical scanning system with position marking is limited and hence also the resolution of the image obtained. Moreover, it has already been stated that displacement inside tubular objects is difficult to achieve, which also makes the marking of the position of the testing device difficult.